Well, since I have Hozan pedal taps that have a long gradual taper, I would just chase the threads on my chosen crank until the pedal goes in. Snug is good; even needing a longish pedal wrench to tighten it would be acceptable to me (not a cheater bar mind you, just a regular pedal wrench).

On my pedal taps, usually going about halfway up the taper is good enough, but if I had to run the tap all the way through, tapping it to the maximum size these taps can make, is still within tolerance. Some other pedal taps are that way too, I believe Eldi have that feature?

If these pedals are from the '70s, unless they are truly NOS (possible but very rare), then this tight one has been in a crank before. Can you say anything about their provenance? My guess is the tight one is just "on the tight side", always has been. Have you tried just reefing on it? With anti-sieze compound maybe or just grease. Show it who's boss!

I would consider that far better than trying to file the threads (precise, evenly all the way down) with a triangle file, that's just not gonna happen no matter how good you are at filing. Especially since the file will quickly dull from the chrome.